It's going to be almost impossible for Britain to decide when to trigger Article 50

The decision concerning when the United Kingdom should trigger
Article 50 — the mechanism by which it will formally begin its
exit from the European Union — will be nigh on impossible, due to
a series of major political events occurring in Europe over the
coming years.

In a note from UBS' Chief Investment Office, titled "Brexit means
...?" — economist Dean Turner, strategist Caroline Simmons, and
analyst Christof Koumoudos, argue that a packed political
calendar, particularly during 2017, will muddy the waters for the
Conservative government in deciding when it wants to leave the
EU.

Regardless of the European Union's commitment to so-called
"ever closer union" nation states are more likely to be
self-interested and put their own health before that of others.
Therefore, if Britain triggers Article 50 during this period, it
isn't certain that negotiations will get the full attention
of the eurozone's three largest economies, as they will
focus on their domestic issues. That is likely something Prime
Minister Theresa May will want to avoid.

As Turner, Simmons, and Koumoudos argue (emphasis ours):

"There is much speculation that the PM will come under pressure
from the Brexiteers in her own party to trigger Article 50 early
next year. While this may satisfy some in her party, many are
aware that delaying the negotiations may be a preferable option.

"It could be argued that it makes sense to postpone triggering
Article 50 until these elections have passed [France and
Germany's general elections], because the UK would then have a
better idea of whom it will be negotiating with. Furthermore,
negotiating with the UK may become a secondary concern
for EU incumbents to winning an election, so valuable negotiating
time in the window could be lost."

UBS

To avoid these problems, May and her government could consider
delaying the triggering of Article 50 until after the German
election in September or October. However, that would cause
problems towards the end of the UK's two-year negotiation period
to leave the EU. Here's UBS once again (emphasis ours):

"A delay until after the German election takes place could pose
its own problems. For example, the next elections to the European
Parliament are due to be held in May 2019. If Article 50
is triggered after next June, in theory, the UK would still be an
EU member during these elections and could be required to elect
MEPs. On the face of it, this situation seems less than
optimal for the UK and the EU if the UK is on course to leave the
bloc shortly thereafter."

UBS notes that a further spanner in the works could come from
negotiations within the EU over its next budget:

"A further consideration is the EU budget negotiations. The
current settlement finishes in 2020, so it would be simpler for
the EU if the UK were gone before the new budget comes into play.
And then there is the next UK general election, scheduled for May
2020. It is unlikely that the PM would want to contest this while
still exiting the EU."

The Financial Times reported on Wednesday that at a
meeting in New York earlier this week banks asked for a "long
lead time" of up to two years to prepare for Brexit and a period
of transition for them to adjust to the new world.